The present invention relates to the field of optics, in particular to an apparatus and method for controlling an optical beam. More specifically, the invention concerns an optical beam shaper and a method for spatial redistribution of inhomogeneous beam. The invention may find application in laser optics, e.g., in shaping and collimation of beams emitted by laser diodes.
A beam emitted practically by any light source has in its cross section two mutually orthogonal directions where the difference in angular divergences of the beam has a maximum value. Conventionally, the direction with the maximal divergence is known as a fast axis, and the one with the minimal divergence is known as a slow axis. For better understanding of this definition, we can refer to FIG. 1, which is a schematic view of a cross section of a beam B with projections B1 and B2 of this beam in two mutually perpendicular planes X-Z and Y-Z. In other words, FIG. 1 shows decomposition of beam image B into two mutually perpendicular projections. It can be seen that the divergence angle xcfx86 of the beam projection B1 in plane Y-Z is greater than the divergence angle "psgr" of the beam projection B2 in plane X-Z. According to the definition given above, axis Y is a fast axis and axis X is a slow axis.
The above phenomenon creates problems in some optical devices where homogeneity of a beam in mutually perpendicular directions of its cross section is a critical factor. One such device is a laser diode which lately finds a very wide application in the fields of communication, materials processing, medical instrumentation, etc.
A laser diode is a light-emitting diode designed to use stimulated emission to form a coherent-light output. This laser has a very high efficiency (Q), is compact in design, but has a very high angular divergence in both fast and slow axes. Therefore, in such devices as laser diodes, and especially in linear arrays of laser diodes, the aforementioned orthogonal difference in divergences is especially noticeable and undesirable.
Heretofore many attempts have been made to solve the above problem in laser diodes and laser diode arrays. For example, U.S. Pat. No. 5,268,922 issued in 1993 to J.-C. Fouere and C. Metreaud discloses a simple optical collimating device for a single laser diode in the form of a single aspheric lens assembled integrally with a laser diode. A disadvantage of such a device is that in order to compensate for divergence difference in orthogonal axes of the beam cross section, the aforementioned aspheric lens should have a complicated custom design. Furthermore, the device of U.S. Pat. No. 5,268,922 is not applicable to laser diode arrays because of even higher spatial angular anisotropy and partial overlapping of beams emitted by adjacent diodes in the same plane.
U.S. Pat. No. 5,636,059 issued in 1997 to J. Snyder discloses an assembly of two aspheric, e.g., cylindrical, lenses with mutually perpendicular generatrices of refractive surfaces. Each lens functions for a separate axis, i.e., one lens reshapes the beam in the fast axis, while the other reshapes the beam in the slow axis. A similar system may consist of two reflective surfaces, e.g., mirrors, similarly located with respect to each other. Although such a system differentiates beam reshaping functions along different axes, it has a macroscopic, i.e., extended size and therefore present a problem for implementation for a matrix-type arrangements of light sources, particularly for those with small steps.
U.S. Pat. No. 5,056,881 issued in 1991 to Terry Bowen, et al. describes an assembly of a laser diode and at least one optical holographic element located at the output of the laser diode. This system circularizes the beam, collimates it, and removes chromatic aberrations. In order to ensure sufficient power compatibility, the holographic element of such a system should be manufactured from a very durable and energetically efficient material such as quartz, which makes the system as a whole relatively expensive. If, on the other hand, the system employs inexpensive, e.g., plastic replicas for the holographic element, it would have practical applications limited only to low-power sources. Furthermore, similar to the system of U.S. Pat. No. 5,636,059, the system with holographic elements is inapplicable to matrix-type sources.
U.S. Pat. No. 4,609,258 issued in 1986 to Iwao Adachi, et al. discloses a collimating system for laser diodes which utilizes a prismatic-type collimator. A disadvantage of this system is that it generates chromatic aberrations inherent in any prismatic systems. Despite the fact that the system itself consists of many components, compensation of the aforementioned aberrations requires the use of additional optical components. As a result, the system has increased overall dimensions.
U.S. Pat. No. 5,541,774 issued in 1996 to R. Blankenbecler describes so-called gradient optical elements. These optical elements can replace various cylindrical, conical, and other aspheric elements use for collimating and beam reshaping. Such optical systems are compact, compatible with matrix-type light sources, but complicated in structure and expensive to manufacture. However, the range of commercially available materials is limited, and therefore gradient optical elements can be manufactured with limitations dictated by wavelengths and output power of the light sources compatible with such optical systems. Another disadvantage of gradient optical elements in light of their application to beam shaping is that they have a limited range of the refractive index variation, which sometimes is insufficient for precise reshaping of the light beam.
U.S. Pat. No. 5,825,551 issued in 1998 to William A. Clarkson discloses a beam shaper utilizing a principle of multiple re-reflection in the system of two parallel reflective surfaces (including the case of total internal reflection). A main disadvantage of such a system is interference of reflected beams which causes spatial modulation of radiation resulting in its inhomogeneity.
Another similar system is described in U.S. Pat. No. 5,808,323 issued in 1998 to Werner Spaeth, et al. This system consists of a cylindrical lens common for a line of photo diodes and two mirrors. The use of a cylindrical lens introduces into the system all disadvantages described above with regard to the systems utilizing aspheric elements. Furthermore, the use of a single cylindrical lens for the entire strip of the diodes does not prevent the adjacent beams from interference and does not allow individual adjustment of beams emitted by individual light sources.
The above disadvantages are partially solved in a fault tolerant optical system described in U.S. Pat. No. 5,369,659 issued in 1994 to Horace Furumoto, et al. The system consists of the following elements arranged in sequence: a laser diode array, two lenslet arrays (collimating and correcting), and directing and focusing optics assembly. However, this system comprises a macroscopic workbench which collimates and corrects individual beams as a whole without addressing the aforementioned fast and slow axes individually, i.e., without separate adjustment of beam divergence in the aforementioned directions. Thus, such a system will not compensate for faults resulting from non-uniform divergence of the beam in the directions of slow and fast axes. This system rather differentiates two functions of the beam shaper, i.e., one lens array is used for correcting the optical faults where the second lens array performs fill-factor enhancement. Another disadvantage of the sytem of U.S. Pat. No. 5,369,659 is that it consists of a plurality of individual lenses produced, e.g., by laser milling. In other words, each array has a composite structure and consists of a plurality of individually manufactured or processed lenses. Moreover, as is stated in the aforementioned U.S. patent, in the manufacturing process with the use of laser milling each individual lens is associated with an individual laser. Thus, the manufacturing process is complicated, expensive, time-consuming, and may involve custom design. The device of U.S. Pat. No. 5,369,659 cannot be produced in a single manufacturing step such as molding or etching.
It is an object of the present invention to provide an optical beam shaper for spatial redistribution of inhomogeneous beam in mutually perpendicular directions of the beam cross section. It is another object is to provide an optical beam shaper of the aforementioned type which has a simple universal design, is easy to manufacture, is applicable to microscopic light source arrays, including matrix-type arrangements of laser diodes with small steps, is free from limitations inherent in beam shapers with holographic elements, does not generate chromatic aberrations, may constitute a single part, e.g., molded from an optical material, has small overall dimensions, is free of limitations dictated by wavelengths and output power of the light sources, allows a wide range of the refractive index variations, is free from interference of individual beams, and facilitates individual adjustment of beams emitted by individual light sources.
The invention relates to an optical beam shaper and a method for spatial redistribution of inhomogeneous beam based on a principle of separate reorientation of beam components along a slow axis and a fast axis. The device is applicable to coherent light sources, e.g., in the form of arrays, including matrix-type arrangements of laser diodes with small steps. One embodiment relates to a beam shaper in which arrays of shaping elements for the slow axis and the fast axis are made in the form of two separate units. In the beam shaper of the second embodiment, shaping element arrays for the slow axis and the fast axis are formed on opposite sides of an integral unit. In the third embodiment, an array of shaping elements for one axis is formed on one side of an integral unit, whereas the beam-shaping element for the second axis is made on the opposite side of the integral block in the form of a single concave lens. The resulting pattern of the beams obtained at the output of the device can be reshaped into any desired configuration. For example, the light emitted by a linear array of laser diodes can be reshaped into a compact pattern suitable for entering an input of an optical fiber cable. The device of the invention can be used as a collimator or a partial collimator, e.g., for laser diode strips.